27.7B: Stages of Labor
Vaginal delivery childbirth has three distinct phases: dilation of the cervix, delivery of the infant, and delivery of the placenta.
- Differentiate among the stages of labor
Key Points
- The first stage of labor begins when the effaced (thinned) cervix is 3 cm dilated. During effacement, the cervix is pulled upward during contractions. Full dilation occurs when the cervix is wide enough to allow passage of the baby’s head, about 10 cm for a full-term infant.
- The duration of labor varies widely, but the active phase averages some 20 hours for women giving birth to their first child (primiparae) and 8 hours for women who have previously given birth (multiparae).
- The second stage begins when the cervix is fully dilated, ending when the baby is born. Fetal movement through the birth canal is assisted by the additional maternal efforts of bearing down or pushing. The fetal head is seen to crown as the labia part, and the baby is born.
- The period from just after the fetus is expelled until just after the placenta is expelled is called the third stage of labor, which lasts, on average, 10–12 minutes.
- A fourth stage of labor may refer to the hour immediately after delivery of the placenta, or may refer to the weeks following delivery. It is the time in which the mother’s body returns to a nonpregnant state.
Key Terms
- multiparae : Women who have given birth to more than one viable fetus.
- Ferguson reflex : An example of positive feedback and the female body’s response to pressure application in the cervix or vaginal walls. Upon application of pressure, oxytocin is released and uterine contractions are stimulated (which will in turn increase oxytocin production, and hence, increase contractions even more), until the baby is delivered.
- primiparae : Women who have given birth to only one child, or who are giving birth for the first time.
The infant’s head and shoulders must go through a specific sequence of maneuvers in order to pass through the ring of the mother’s pelvis. The six phases of a typical vertex (head-first presentation) delivery are:
- Engagement of the fetal head in the transverse position.
- Descent and flexion of the fetal head.
- Internal rotation of the fetal head so that the baby’s face is towards the mother’s rectum.
- Delivery by extension (the fetal head passes out of the birth canal).
- Restitution: The fetal head turns through 45 degrees to restore its normal relationship with the shoulders, which are still at an angle.
- External rotation: The shoulders repeat the corkscrew movements of the head, which can be seen in the final movements of the fetal head.
The Bishop score defines several factors that midwives and physicians use to assess the laboring mother’s progress.
The score is used to predict whether the mother is likely to spontaneously progress into the second stage of labor and whether induction of labor will be required. It has also been used to assess the odds of spontaneous preterm delivery.
Stages of Labor
First Stage (Active Phase)
The first stage of labor classically starts when the effaced (thinned) cervix is 3 cm dilated, although there is variation as some women may or may not have active contractions prior to reaching this point. The onset of actual labor is defined when the cervix begins to progressively dilate. Rupture of the membranes or a bloody discharge may or may not occur at or around this stage.
Tterine muscles form opposing spirals from the top of the upper segment of the uterus to its junction with the lower segment. During effacement, the cervix becomes incorporated into the lower segment of the uterus. During a contraction, these muscles shorten the upper segment, drawing upwards the lower segment in a gradual expulsive motion.
The presenting fetal part is then permitted to descend. Full dilation is reached when the cervix has widened enough to allow passage of the baby’s head, around 10 cm dilation for a term baby. The duration of labor varies widely, but the active phase averages some 20 hours for women giving birth to their first child (primiparae), and 8 hours for women who have already given birth (multiparae).
Active phase arrest is defined, in a primigravid woman, as the failure of the cervix to dilate at a rate of 1.2 cm/hr over a period of at least two hours. This definition is based on Friedman’s Curve, which plots the typical rate of cervical dilation and fetal descent during active labor. Some practitioners may diagnose failure to progress, and consequently, perform a caesarean.
Second Stage (Fetal Expulsion)
The second stage begins when the cervix is fully dilated and ends when the baby is born. As pressure on the cervix increases, the Ferguson reflex increases uterine contractions.
At the beginning of the normal second stage, the head is fully engaged in the pelvis: the widest diameter of the head has passed below the level of the pelvic inlet. The fetal head then continues descending into the pelvis, below the pubic arch, and out through the vagina.
This is assisted by the additional maternal efforts of bearing down or pushing. The fetal head is seen to crown as the labia part. At this point, the woman may feel a burning or stinging sensation. The complete expulsion of the baby signals the successful completion of the second stage of labor.
The second stage of birth will vary by factors including parity, fetal size, anesthesia, or the presence of infection. Longer labors are associated with declining rates of spontaneous vaginal delivery and increasing rates of infection, perineal laceration, obstetric hemorrhage, as well as need for intensive care of the neonate.
Third Stage (Placental Delivery)
The third stage of labor is the period from just after the fetus is expelled until just after the placenta is expelled. The average time from delivery of the baby until complete expulsion of the placenta is estimated to be 10–12 minutes.
Placental expulsion can be managed actively, by giving a uterotonic, such as oxytocin, along with appropriate cord traction and fundal massage to assist in delivering the placenta by a skilled birth attendant. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. The umbilical cord is routinely clamped and cut in this stage, but it would normally close naturally even if not clamped and cut.
When the amniotic sac has not ruptured during labour or pushing, the infant can be born with the membranes intact. This is referred to as being born in the caul. The caul is harmless and its membranes are easily broken and wiped away. With the advent of modern interventive obstetrics, the artificial rupture of the membranes has become common, so babies are rarely born in the caul (en-caul birth).
Fourth Stage (Postpartum Period)
The fourth stage of labor is a term used in two different senses:
- It can refer to the immediate puerperium, or the hours immediately after delivery of the placenta.
- It can be used in a more metaphorical sense to describe the weeks following delivery.
Birth. : A sequence of images showing the stages of ordinary childbirth.